Ceramic heating elements

ABSTRACT

New methods are provided or manufacture ceramic resistive igniter elements that include sintering of the elements in the absence of substantially elevated pressures. Ceramic igniters also are provided that are obtainable from fabrication methods of the invention.

The present applications claims the benefit of U.S. provisionalapplication No. 60/798,266 filed May 4, 2006, incorporated by referencedherein in its entirety.

BACKGROUND

1. Field of the Invention

In one aspect, the invention provides new methods for manufactureceramic heating elements that include substantially pressurelesssintering of the formed green igniter element. Igniter elements also areprovided, including such elements obtainable from fabrication methods ofthe invention.

2. Background

Ceramic materials have enjoyed great success as igniters in e.g.gas-fired furnaces, stoves and clothes dryers. Ceramic igniterproduction includes constructing an electrical circuit through a ceramiccomponent a portion of which is highly resistive and rises intemperature when electrified by a wire lead. See, for instance, U.S.Pat. Nos. 6,582,629; 6,278,087; 6,028,292; 5,801,361; 5,786,565;5,405,237; and 5,191,508.

Typical igniters have been generally rectangular-shaped elements with ahighly resistive “hot zone” at the igniter tip with one or moreconductive “cold zones” providing to the hot zone from the opposingigniter end. One currently available igniter, the Mini-Igniter™,available from Norton Igniter Products of Milford, N.H., is designed for12 volt through 120 volt applications and has a composition comprisingaluminum nitride (“AlN”), molybdenum disilicide (“MoSi₂”), and siliconcarbide (“SiC”).

Igniter fabrication methods have included batch-type processing where adie is loaded with ceramic compositions of at least two differentresistivities. The formed green element is then densified (sintered) atelevated temperature and pressure. See the above-mentioned patents. Seealso U.S. Pat. No. 6,184,497.

While such fabrication methods can be effective to produce ceramicigniters, the protocols can present inherent limitations with respect tooutput and cost efficiencies.

It thus would be desirable to have new heating element systems. It wouldbe particularly desirable to have new methods for producing ceramicheating elements. It also would be desirable to have more efficientproduction methods.

SUMMARY OF THE INVENTION

In one aspect, new ceramic articles are provided which are formed fromone or more ceramic powders that have a mean particle size of about 2.5microns or less.

We have found that ceramic articles made from such small size ceramicmaterials can be densified under significantly more mild conditions,including under reduced pressures relative to prior procedures.

In another aspect, ceramic articles are provided that are fabricated bytreatment of the green state ceramic article by multiple, increasingpressures. Preferably, the ceramic article is treated at a firstpressure and then treated at a second pressure which is higher than thefirst pressure. Preferably, the multi-pressure densification isconducted with use of gas-pressure sintering.

We have found that the multiple-stage pressure treatments can provide ahighly dense article (e.g. at least 96, 97, 98 or 99 dense percent)ceramic article under quite mild conditions. For instance, the firstpressure treatment suitably may be at about 1000 psi or 500 psi or lessand the second pressure treatment may be at 4000 psi or less.Significantly lower pressures also have yielded highly dense articles,such as a first pressure of about 200 psi or less or 150 psi or less anda second pressure treatment of about 3000 psi or less, 2000 psi or lessor 1500 psi or less.

In particularly preferred aspects of the invention, ceramic compositionsare utilized that comprise one or more metal oxides such as alumina.Preferably, the one or more one or more metal oxides have a small meanparticle size as disclosed herein. Particularly preferred are ceramiccompositions that comprise alumina with small mean particle size asdisclosed herein, such as 2.5 microns or less, 2 microns or less, 1.5microns or less or 1 micron or less.

In a further aspect of the invention, ceramic compositions are densifiedin the absence of a so-called sintering aid. Sintering aid additiveshave included rare earth oxides, such as yttria (yttrium oxide), agadolinium material (e.g. a gadolinium oxide or Gd₂O₃), a europiummaterial (e.g. a europium oxide or Eu₂O₃), a ytterbium material (e.g. aytterbium oxide or Yb₂O₃), or a lanthanum material (e.g. lanthanum orLa₂O₃).

Particularly preferred fabrication methods of the invention includeforming a ceramic igniter element that comprises one or more smallparticle size ceramic materials as discussed above and then hardeningthrough a two-stage pressure treatment as discussed above. Suitably,hardening is conducted under elevated temperatures such as in excess of1400° C., more typically in excess of 1600° C. such as at least 1700° C.or 1800° C. Preferably, the sintering is conducted under an inertatmosphere, e.g. in an atmosphere of an inert gas such as argon ornitrogen.

Preferably, the hardening treatment provides a ceramic element that isat least 95 percent dense, more preferably a ceramic element that is atleast 96, 97, 98 or 99 percent dense. The hardening process whichincludes the noted elevated temperatures is conducted for a timesufficient to achieve such densities, which may be several hours ormore.

Particular ceramic compositions and method of forming the green ceramicelement may be utilized to facilitate producing a dense ceramic elementin the absence of substantially elevated pressures.

More specifically, preferred ceramic compositions employed to form aceramic element may be at least substantially free or completely free ofsilicon carbide, or other carbide material. As referred to herein, aceramic composition is at least substantially free of silicon carbide orother carbide material if it contains less than 10 volume percent ofsilicon carbide or other carbide material based on total volume of theceramic composition, more typically less than about 9, 8, 7, 6, 5,4, 3,2, 1 or 0.5 volume percent based on total volume of the ceramiccomposition.

For sintering a ceramic element that comprises alumina, preferablysintering of the element is conducted in an atmosphere that is at leastsubstantially free of nitrogen (e.g. less than 5 volume % nitrogen basedon total atmosphere), or more preferably at least essentially free ofnitrogen (e.g. less than 2 or 1 volume % nitrogen based on totalatmosphere), or more preferably completely free of nitrogen. Forinstance, sintering may be conducted in an Argon atmosphere.

For sintering a ceramic element that comprises AlN, preferably sinteringof the element is conducted in an atmosphere that contains at least somenitrogen, e.g. at least about 5 volume percent of nitrogen (i.e. atleast 5 volume % nitrogen based on total atmosphere), or higher levelssuch as at least about 10 volume percent of nitrogen (i.e. at least 10volume. % nitrogen based on total atmosphere).

It also may be preferred to form the ceramic elements through aninjection molding process. As typically referred to herein, the term“injection molded,” “injection molding” or other similar term indicatesthe general process where a material (here a ceramic or pre-ceramicmaterial) is injected or otherwise advanced typically under pressureinto a mold in the desired shape of the ceramic element followed bycooling and subsequent removal of the solidified element that retains areplica of the mold.

In injection molding formation of heating elements of the invention, aceramic material (such as a ceramic powder mixture, dispersion or otherformulation) or a pre-ceramic material or composition may be advancedinto a mold element.

In suitable fabrication methods, an integral igniter element havingregions of differing resistivities (e.g., conductive region(s),insulator or heat sink region and higher resistive “hot” zone(s)) may beformed by sequential injection molding of ceramic or pre-ceramicmaterials having differing resistivities.

Thus, for instance, a base element may be formed by injectionintroduction of a ceramic material having a first resistivity (e.g.ceramic material that can function as an insulator or heat sink region)into a mold element that defines a desired base shape such as a rodshape. The base element may be removed from such first mold andpositioned in a second, distinct mold element and ceramic materialhaving differing resistivity—e.g. a conductive ceramic material—can beinjected into the second mold to provide conductive region(s) of theigniter element. In similar fashion, the base element may be removedfrom such second mold and positioned in a yet third, distinct moldelement and ceramic material having differing resistivity—e.g. aresistive hot zone ceramic material—can be injected into the third moldto provide resistive hot or ignition region(s) of the heating element.

In preferred aspects of the invention, at least three portions of aceramic heating element are injection molded in single fabricationsequence to produce a ceramic component, a so-called “multiple shot”injection molding process where in the same fabrication sequence wheremultiple portions of an igniter element having different resistivityvalues (e.g. hot or highly resistive portion, cold or conductiveportion, and insulator or heat sink portion). In at least certainembodiments, a single fabrication sequence includes sequential injectionmolding applications of a ceramic material without removal of theelement from the element-forming area and/or without deposition ofceramic material to an element member by a process other than injectionmolding.

For instance, in one aspect, a first insulator (heat sink) portion canbe injection molded, around that insulator portion conductive legportions then can be injection molded in a second step, and in a thirdstep a resistive hot or ignition zone can be applied by injectionmolding to the body containing insulator and resistive zones.

In another embodiment, methods for producing a resistive ceramic heatingelement are provided, which include injection molding one or moreportions of a ceramic element, wherein the ceramic element comprisesthree or more regions of differing resistivity.

Fabrication methods of the invention may include additional processesfor addition of ceramic material to produce the formed ceramic element.For instance, one or more ceramic layers may be applied to a formedelement such as by dip coating, spray coating and the like of a ceramiccomposition slurry.

Preferred ceramic elements obtainable by methods of the inventioncomprise a first conductive zone, a resistive hot zone, and a secondconductive zone, all in electrical sequence. Preferably, during use ofthe device electrical power can be applied to the first or the secondconductive zones through use of an electrical lead.

Particularly preferred heating elements of the invention will have arounded cross-sectional shape along at least a portion of the heatingelement length (e.g., the length extending from where an electrical leadis affixed to the igniter to a resistive hot zone). More particularly,preferred ceramic heating elements may have a substantially oval,circular or other rounded cross-sectional shape for at least a portionof the igniter length, e.g. at least about 10 percent, 40 percent, 60percent, 80 percent, 90 percent of the igniter length, or the entireigniter length. A substantially circular cross-sectional shape thatprovides a rod-shaped heating element is particularly preferred. Suchrod configurations offer higher Section Moduli and hence can enhance themechanical integrity of the heating element.

Ceramic heating elements of the invention can be employed at a widevariety of nominal voltages, including nominal voltages of 6, 8, 10, 12,24,120, 220, 230 and 240 volts.

The heating elements of the invention are useful for ignition in avariety of devices and heating systems. More particularly, heatingsystems are provided that comprise a sintered ceramic igniter element asdescribed herein. Specific heating systems include gas cooking units,heating units for commercial and residential buildings, including waterheaters.

As referred to herein, the term “ceramic material” includes materialsboth prior to and after sintering processes. For instance, alumina,Mo₂Si₂, SiC, AlN and other materials referred to herein are consideredceramic materials including in the pre-sintered state of thosematerials.

Other aspects of the invention are disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show top and bottom views respectively of a heatingelement of the invention;

FIG. 2A shows a cut-away view along line 2A-2A of FIG. 1A; and

FIG. 2B shows a cut-away view along line 2B-2B of FIG. 1A.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, new ceramic articles are provided which are formedfrom one or more ceramic powders that have a mean particle size of about2.5 microns or less, more preferably a mean particle size of about 2microns or less, or 1.5, 1.25 or 1 micron or less. Such ceramicmaterials typically have a mean particle size of at least about 0.2,0.3, 0.4 or 0.5 microns.

In preferred ceramic compositions, at least a major portion (e.g.greater than 50, 60, 70, 80 or 90 weight percent) of a specified ceramicmaterial will have a small particle size as disclosed herein. Morepreferred, the entire portion of the specified ceramic material willhave such a small particle size. For example, if a ceramic compositionis indicated to include alumina having a mean particle size of 2 micronsor less, preferably at least a major portion (such as greater than 50,60, 70, 80 or 90 weight percent) of the alumina utilized in the ceramiccomposition will have a mean particle of 2 microns or less, and morepreferably the entire portion of alumina present in the ceramiccomposition will have a mean particle size of 2 microns or less.

As discussed herein, ceramic compositions employed to produce heatingelements of the invention may suitably comprise two, three or moredistinct materials such as Al₂O₃, AlN, Mo₂Si₂, SiC, and the like.Suitably, one or more of such distinct materials may be employed insmall mean particle size as disclosed herein. However, in certainembodiments, not all materials of a ceramic compositions need to beemployed in such mean small particle sizes. In this aspect of theinvention, at least one material of a multiple-material composition isof such small mean particle size, but more than one or all materials ofa multiple-material composition may have such small mean particle sizesif desired.

As discussed above, in certain embodiments, use of a small mean particlesize metal oxide such as Al₂O₃ may be particularly preferred.

Without being bound by any theory, it is believed that use of suchsmaller mean size particle materials can facilitate reduced pressuresintering of the formed green state heating element.

In another aspect, as discussed above, new methods are now provided forproducing ceramic igniter elements that include hardening (densifiying)of a formed green ceramic element under reduced elevated pressures.

In this aspect, ceramic articles are provided that are fabricated bytreatment of the green state ceramic article by multiple, increasingpressures. Preferably, the ceramic article is treated at a firstpressure and then treated at a second pressure which is higher than thefirst pressure.

For at least certain applications, the first and second pressuretreatments differ by at least 500 psi, more preferably by at least 1000psi, 2000 psi or 2500 psi.

For at least certain applications, the first pressure treatment suitablymay be at about 3,000 psi or less, 2000 psi or less, 1000 psi or less,500 psi or less, or 200 psi or less, and the second pressure treatmentmay be at 6000 psi or less, 5000 psi or less, 4000 psi or less, 3000 psior less, 2000 psi or less, 1500 psi or less or 1000 psi or less.

For at least certain applications, the first pressure treatment and thesecond pressure treatment each will not exceed 5000 psi.

Other pressures also may be employed for the first and second pressuretreatments provided the first pressure treatment is at a lower levelthan the second pressure treatment.

Again, without wishing to be bound by theory, it is believed a firstlower pressure treatment can provide an initial densification thatavoids entrapped gases within the article. Once porosity issignificantly closed by the first pressure treatment, higherdensifications can be achieved in the elevated second pressuretreatment.

Preferably, the multi-pressure densification is conducted with use ofgas-pressure sintering. Commercial gas phase sintering ovens may beemployed. Preferably, sintering is conducted under an inert atmosphere,such as a nitrogen or argon atmosphere.

As discussed above, in a further aspect of the invention, ceramiccompositions are densified in the absence of a so-called sintering aid.

As discussed above, ceramic elements may be preferably formed byinjection molding techniques. Thus, for instance and as discussed above,a base element may be formed by injection introduction of a ceramicmaterial having a first resistivity (e.g. ceramic material that canfunction as an insulator or heat sink region) into a mold element thatdefines a desired base shape such as a rod shape. The base element maybe removed from such first mold and positioned in a second, distinctmold element and ceramic material having differing resistivity—e.g. aconductive ceramic material—can be injected into the second mold toprovide conductive region(s) of the heating element. In similar fashion,the base element may be removed from such second mold and positioned ina yet third, distinct mold element and ceramic material having differingresistivity—e.g. a resistive hot zone ceramic material—can be injectedinto the third mold to provide resistive hot or ignition region(s) ofthe heating element.

Alternatively, rather than such use of a plurality of distinct moldelements, ceramic materials of differing resistivitities may besequentially advanced or injected into the same mold element. Forinstance, a predetermined volume of a first ceramic material (e.g.ceramic material that can function as an insulator or heat sink region)may be introduced into a mold element that defines a desired base shapeand thereafter a second ceramic material of differing resistivity may beapplied to the formed base.

Ceramic material may be advanced (injected) into a mold element as afluid formulation that comprises one or more ceramic materials such asone or more ceramic powders.

For instance, a slurry or paste-like composition of ceramic powders maybe prepared, such as a paste provided by admixing one or more ceramicpowders with an aqueous solution or an aqueous solution that containsone or more miscible organic solvents such as alcohols and the like. Apreferred ceramic slurry composition for extrusion may be prepared byadmixing one or more ceramic powders such as MoSi₂, Al₂O₃, and/or AlN ina fluid composition of water optionally together with one or moreorganic solvents such as one or more aqueous-miscible organic solventssuch as a cellulose ether solvent, an alcohol, and the like. The ceramicslurry also may contain other materials e.g. one or more organicplasticizer compounds optionally together with one or more polymericbinders.

A wide variety of shape-forming or inducing elements may be employed toform an igniter element, with the element of a configurationcorresponding to desired shape of the formed igniter. For instance, toform a rod-shaped element, a ceramic powder paste may be injected into acylindrical die element. To form a stilt-like or rectangular-shapedigniter element, a rectangular die may be employed.

After advancing ceramic material(s) into a mold element, the definedceramic part suitably may be dried e.g. in excess of 50° C. or 60° C.for a time sufficient to remove any solvent (aqueous and/or organic)carrier.

The examples which follow describe preferred injection molding processesto form an igniter element.

Referring now to the drawings, FIGS. 1A and 1B shows a suitable heatingelement 10 of the invention.

As can be seen in FIG. 1A, igniter 10 includes a central heat sink orinsulator region 12 which is encased within region(s) of differingresistivity, namely conductive zones 14 in the proximal portion 16 whichbecome more resistive where in igniter proximal portion 18 the regionhas a comparatively decreased volume and thus can function as resistivehot zone 20.

FIG. 1B shows igniter bottom face with exposed heat sink region 12.

Cross-sectional views of FIGS. 2A and 2B further depict heating element10 which includes conductive zones 14A and 14B in igniter proximalregion 16 and corresponding resistive hot zone 20 in igniter distal zone18.

In use, power can be supplied to heating element 10 (e.g. via one ormore electrical leads, not shown) into conductive zone 14A whichprovides an electrical path through resistive ignition zone 20 and thenthrough conductive zone 14B. Proximal ends 14 a of conductive regions 14may be suitably affixed such as through brazing to an electrical lead(not shown) that supplies power to the igniter during use. The igniterproximal end 10 a suitably may be mounted within a variety of fixtures,such as where a ceramoplastic sealant material encases conductiveelement proximal end 14 a as disclosed in U.S. Published PatentApplication 2003/0080103. Metallic fixtures also maybe suitably employedto encase the heating element proximal end.

As discussed above, and exemplified by heating element 10 of FIGS. 1A,1B, 2A and 2B, at least a substantial portion of the igniter length hasa rounded cross-sectional shape along at least a portion of the heatingelement length, such as length x shown in FIG. 1B. Igniter 10 of FIGS.1A, 1B, 2A and 2B depicts a particularly preferred configuration whereheating element 10 has a substantially circular cross-sectional shapefor about the entire length of the heating element to provide arod-shaped heating element element. However, preferred systems alsoinclude those where only a portion of the igniter has a roundedcross-sectional shape, such as where up to about 10, 20, 30, 40, 50, 60,70 80 or 90 of the heating element length (as exemplified by heatingelement length x in FIG. 1B) has a rounded cross-sectional shape; insuch designs, the balance of the heating element length may have aprofile with exterior edges.

Heating element of a variety of configurations may be fabricated asdesired for a particular application. Thus, for instance, to provide aparticular configuration, an appropriate shape-inducing mold element isemployed through which a ceramic composition (such as a ceramic paste)may be injected.

Dimensions of heating elements of the invention may vary widely and maybe selected based on intended use of the heating element. For instance,the length of a preferred heating element (length x in FIG. 1B) suitablymay be from about 0.5 to about 5 cm, more preferably from about 1 about3 cm, and the heating element cross-sectional width may suitably be fromabout (length y in FIG. 1B) suitably may be from about 0.2 to about 3cm.

Similarly, the lengths of the conductive and hot zone regions also maysuitably vary. Preferably, the length of a first conductive zone (lengthof proximal region 16 in FIG. 1A) of a heating element of theconfiguration depicted in FIG. 1A may be from 0.2 cm to 2, 3, 4, or 5more cm. More typical lengths of the first conductive zone will be fromabout 0.5 to about 5 cm. The total hot zone electrical path length(length f in FIG. 1A) suitably may be about 0.2 to 5 or more cm.

In preferred systems, the hot or resistive zone of a heating element ofthe invention will heat to a maximum temperature of less than about1450° C. at nominal voltage; and a maximum temperature of less thanabout 1550° C. at high-end line voltages that are about 110 percent ofnominal voltage; and a maximum temperature of less than about 1350° C.at low-end line voltages that are about 85 percent of nominal voltage.

A variety of compositions may be employed to form a heating element ofthe invention. Generally preferred hot zone compositions comprise two ormore components of 1) conductive material; 2) semiconductive material;and 3) insulating material. Conductive (cold) and insulative (heat sink)regions may be comprised of the same components, but with the componentspresent in differing proportions. Typical conductive materials includee.g. molybdenum disilicide, tungsten disilicide, and nitrides such astitanium nitride. Typical insulating materials include metal oxides suchas alumina or a nitride such as AlN and/or Si₃N₄.

As referred to herein, the term electrically insulating materialindicates a material having a room temperature resistivity of at leastabout 10¹⁰ ohms-cm. The electrically insulating material component ofigniters of the invention may be comprised solely or primarily of one ormore metal nitrides and/or metal oxides, or alternatively, theinsulating component may contain materials in addition to the metaloxide(s) or metal nitride(s). For instance, the insulating materialcomponent may additionally contain a nitride such as aluminum nitride(AlN), silicon nitride, or boron nitride; a rare earth oxide (e.g.yttria); or a rare earth oxynitride. A preferred added material of theinsulating component is alumina (Al₂O₃).

As referred to herein, a semiconductor ceramic (or “semiconductor”) is aceramic having a room temperature resistivity of between about 10 and10⁸ ohm-cm. If the semiconductive component is present as more thanabout 45 v/o of a hot zone composition (when the conductive ceramic isin the range of about 6-10 v/o), the resultant composition becomes tooconductive for high voltage applications (due to lack of insulator).Conversely, if the semiconductor material is present as less than about10 v/o (when the conductive ceramic is in the range of about 6-10 v/o),the resultant composition becomes too resistive (due to too muchinsulator). Again, at higher levels of conductor, more resistive mixesof the insulator and semiconductor fractions may be needed to achievethe desired voltage.

As referred to herein, a conductive material is one which has a roomtemperature resistivity of less than about 10⁻² ohm-cm. If theconductive component is present in an amount of more than 35 v/o of thehot zone composition, the resultant ceramic of the hot zone composition,the resultant ceramic can become too conductive. Typically, theconductor is selected from the group consisting of molybdenumdisilicide, tungsten disilicide, and nitrides such as titanium nitride,and carbides such as titanium carbide. Molybdenum disilicide isgenerally preferred.

In general, preferred hot (resistive) zone compositions include (a)between about 50 and about 80 v/o of an electrically insulating materialhaving a resistivity of at least about 10¹⁰ ohm-cm; (b) between about 0(where no semiconductor material employed) and about 45 v/o of asemiconductive material having a resistivity of between about 10 andabout 10⁸ ohm-cm; and (c) between about 5 and about 35 v/o of a metallicconductor having a resistivity of less than about 10⁻² ohm-cm.

As discussed, heating element of the invention contain a relatively lowresistivity cold zone region in electrical connection with the hot(resistive) zone and which allows for attachment of wire leads to theigniter. Preferred cold zone regions include those that are comprised ofe.g. AlN and/or Al₂O₃ or other insulating material; optionalsemiconductor material; and MoSi₂ or other conductive material. However,cold zone regions will have a significantly higher percentage of theconductive materials (e.g., MoSi₂) than the hot zone. A preferred coldzone composition comprises about 15 to 65 v/o aluminum oxide, aluminumnitride or other insulator material; and about 20 to 70 v/o MoSi₂ orother conductive and semiconductive material in a volume ratio of fromabout 1:1 to about 1:3. For ease of manufacture, preferably the coldzone composition is formed of the same materials as the hot zonecomposition, with the relative amounts of semiconductive and conductivematerials being greater.

At least certain applications, heating elements of the invention maysuitably comprise a non-conductive (insulator or heat sink) region,although particularly preferred heating elements of the invention do nothave a ceramic insulator that contacts at least a substantial portion ofthe length of a first conductive zone, as discussed above.

If employed, such a heat sink zone may mate with a conductive zone or ahot zone, or both. Preferably, a sintered insulator region has aresistivity of at least about 10¹⁴ ohm-cm at room temperature and aresistivity of at least 10⁴ ohm-cm at operational temperatures and has astrength of at least 150 MPa. Preferably, an insulator region has aresistivity at operational (ignition) temperatures that is at least 2orders of magnitude greater than the resistivity of the hot zone region.Suitable insulator compositions comprise at least about 90 v/o of one ormore aluminum nitride, alumina and boron nitride

Preferred heating element ceramic materials are disclosed in theexamples which follow.

Heating elements of the invention may be used in many applications,including gas phase fuel ignition applications such as furnaces andcooking appliances, baseboard heaters, boilers, and stove tops. Inparticular, a heating element of the invention may be used as anignition source for stop top gas burners as well as gas furnaces.

In one preferred aspect of the invention, heating elements of theinvention may be configured and/or utilized as resistive igniterselements distinct from heating elements known as glow plugs. Among otherthings, frequently employed glow plugs often heat to relatively lowertemperatures e.g. a maximum temperature of about 800° C., 900° C. or1000° C. and thereby heat a volume of air rather than provide directignition of fuel, whereas preferred igniters of the invention canprovide maximum higher temperatures such as at least about 1200° C.,1300° C. or 1400° C. to provide direct ignition of fuel. Preferredigniters of the invention also need not include gas-tight sealing aroundthe element or at least a portion thereof to provide a gas combustionchamber, as typically employed with a glow plug system. Still further,many preferred igniters of the invention are useful at relatively highline voltages, e.g. a line voltage in excess of 24 volts, such as 60volts or more or 120 volts or more including 220, 230 and 240 volts,whereas glow plugs are typically employed only at voltages of from 12 to24 volts.

Heating elements of the invention also are particularly suitable for usefor ignition where liquid (wet) fuels (e.g. kerosene, gasoline) areevaporated and ignited, e.g. in vehicle (e.g. car) heaters that provideadvance heating of the vehicle.

In other preferred aspects, heating elements are suitably employed asglow plugs, e.g. as an ignition source in a motor vehicle.

Heating elements will be useful for additional specific applications,including as a heating elements for an infrared heater.

The following non-limiting examples are illustrative of the invention.All documents mentioned herein are incorporated herein by reference intheir entirety.

EXAMPLE 1 Igniter Fabrication

The following materials were admixed to provide a conductive compositionfor injection molding fabrication of a heating element: 30 vol % MoSi₂,7 vol % SiC, and 63 vol % Al₂O₃, and based on the weight of ceramicmaterials 2˜3 wt % of polyvinylalcohol and 0.3 wt % of glycerol.

The following materials were admixed to provide an insulator compositionfor injection molding fabrication of a heating element: 10 vol % MoSi₂,90 vol % Al₂O₃, and based on the weight of ceramic materials 2˜3 wt % ofpolyvinylalcohol and 0.3 wt % of glycerol.

The following materials were admixed to provide a resistive hot zonecomposition for injection molding fabrication of a heating element: 25vol % MoSi₂, 5 vol % SiC, and 70 vol % Al₂O₃, and based on the weight ofceramic materials 2˜3 wt % of polyvinylalcohol and 0.3 wt % of glycerol.

In each of the three compositions, the Al₂O₃ had a mean particle size of1.7 microns. No sintering aids such as yttria or other such materialswere included in the compositions.

The above three compositions of differing resisitivity were loaded intoseparate barrels of a co-injection molder. To form the rod-shapedigniter element with internal insulator region of the generalconfiguration shown in FIG. 1 of the drawings, a first shot filled ahalf-cylinder shaped cavity with insulating paste forming the insulatingpaste extruded from the cavity. The part was removed from the firstcavity, placed in a second cavity and a second shot filled the volumebounded by the first shot and the cavity wall core with the conductivepaste. The part was then removed from the second cavity, placed in athird cavity and a third shot filled the top portion of the part withthe resistive (hot zone) paste. The thus molded rod-shaped part was thenpartially debindered at room temperature in an organic solventdissolving out 10 wt % of the added 10-16 wt %. The part was thenthermally debindered in flowing inert gas (N₂) at 300-500° C. for 60hours to remove the remainder of the residual binder.

The debindered rod-shaped part was densified through a two-stage processusing gas-phase sintering. Thus, the rod-shaped part was placed in a gassintering oven which was filled with argon gas at a pressure of 150 psi.The oven was maintained at 1725° C. for about 1.5 hours. The oven wasthen allowed to cool to room temperature and then pressure increased to3000 psi and held at 1725° C. for about 2 hours. The oven was thenallowed to cool to room temperature. The treated rod-shaped part had adensity of greater than 98 percent. The dense element was connected to apower supply of 24 volts and the hot zone attained a temperature ofabout 1300° C.

The invention has been described in detail with reference to particularembodiments thereof. However, it will be appreciated that those skilledin the art, upon consideration of this disclosure, may make modificationand improvements within the spirit and scope of the invention.

1. A resistive ceramic heating element comprising: prior to sintering,one or more ceramic materials having a mean particle size of 2.5 micronsor less.
 2. The heating element of claim 1 wherein the heating elementcomprises prior to sintering one or more metal oxides having a meanparticle size of 2.5 microns or less.
 3. The ceramic heating element ofclaim 1 wherein the heating element comprises prior to sintering aluminahaving a mean particle size of 2.5 microns or less.
 4. The heatingelement of claim 1 wherein the one or more ceramic materials have a meanparticle size of 2 microns or less.
 5. The heating element of claim 1wherein the one or more ceramic materials have a mean particle size of1.5 microns or less.
 6. A method for producing a resistive heatingelement, comprising: treating a ceramic composition at a first pressure;and thereafter treating the ceramic composition at a second pressurethat is greater than the first pressure to thereby densify the ceramiccomposition.
 7. The method of claim 6 wherein prior to treatment at thefirst pressure the ceramic composition comprises one or more ceramicmaterials having a mean particle size of 2.5 microns or less.
 8. Themethod of claim 6 wherein prior to treatment at the first pressure theceramic composition the ceramic composition comprises one or more metaloxides having a mean particle size of 2.5 microns or less.
 9. The methodof claim 6 wherein prior to treatment at the first pressure the ceramiccomposition comprises alumina having a mean particle size of 2.5 micronsor less.
 10. The method of claim 6 wherein the first and secondpressures differ by at least 1000 psi.
 11. The method of claim 6 whereinthe second pressure is about 5000 psi or less.
 12. The method of claim 6wherein the first pressure is about 1000 psi or less.
 13. The method ofclaim 6 wherein the first pressure is about 250 psi or less.
 14. Themethod of claim 6 wherein the first and second pressures are applied asa gas phase sintering process.
 15. The method of claim 6 wherein theceramic igniter element is formed of a composition that has less than 10volume percent silicon carbide.
 16. The method of claim 6 wherein theceramic element comprises two or more regions of differing resistivity.17. The method of claim 6 wherein the ceramic element comprises three ormore regions of differing resistivity.
 18. A ceramic igniter elementobtainable by the method of claim
 6. 19. A method of igniting gaseousfuel, comprising applying an electric current across an igniter of claim18.
 20. A heating apparatus comprising an igniter of claim 18.